It was the sort of theatricality usually associated with Trump, the bombastic showman. But bravado is clearly catching in the Republican primary race, where with only four states decided, there is already mounting hysteria that Donald J Trump’s hostile takeover of the Republican party is a done deal.

Jeb Bush is long gone, and for the party grandees, fresh-faced Rubio, the 44-year-old Florida senator, is all that stands in the way of Trump or the almost equally unpalatable Texas senator Ted Cruz.

Can Rubio be the Trump slayer?

Until now, Rubio had shied away from confrontation with the boorish frontrunner. But with the clock ticking and his back against the wall, Rubio took him on in the final televised debate before this week’s Super Tuesday contest in 12 states which could put Trump way out in front.

Rubio shed his nice-guy image in the debate in Houston, tirelessly needling Trump on virtually every aspect of his candidacy: his record of hiring foreign workers for his grand real estate projects; on disguising himself as a conservative, citing Trump’s prior support for the family planning group Planned Parenthood; and more fundamentally, as a man lacking in substance.

“You say the same thing every night,” Rubio said, standing alongside Trump on the stage. “Everyone’s dumb, I’m going to make America great again, I’m winning in the polls, lines around the states, every night.”

It was a performance that grabbed most of the night’s headlines, even as reporters and pundits wondered why Rubio had not unveiled this version of himself sooner. The senator’s team insisted the timing came not from desperation but was a concerted calculation.

“When you have 16 or 17 candidates in a race, you’re playing seven-dimensional chess, and if you take on one candidate aggressively, you don’t necessarily accrue the benefit of that,” Rubio’s senior strategist Todd Harris told reporters in the so-called spin room.

“We felt like this was the exact right time to show the Republican base that Marco Rubio is the one candidate best able to take on and take out Donald Trump.”

The strange truth, in a truly unusual race, is that at a surface level everything has gone precisely to plan for Rubio.

First, he disposed of Bush, whose exit from the race after the South Carolina primary prompted a rush of endorsements for Rubio and commitments from high-dollar donors. He has also begun to inch ahead of Cruz, the other Cuban-American first-term senator, campaigning from the evangelical, social conservative right.

The trouble is, the advance plan map to the nomination made no mention of the uncharted Trump, who steamrolls on, poised for a near-sweep on Super Tuesday, a day that could prove decisive for both parties.

On the Republican side, 595 delegates – roughly a quarter of the total required to clinch the nomination – are at stake across a dozen states. For Democrats, a once-competitive race looks more predictable as Hillary Clinton is expected to score big victories over Bernie Sanders and cement her status as the presumptive nominee.

Whether buzzing across the early voting states or the American south, Rubio has defiantly stated what many Democrats privately acknowledge: he is the Republican who can defeat Clinton.

Invoking his humble beginnings as the son of working-class immigrants, Rubio vows to reach voters long forgotten by the Republican party.

“When I’m our nominee, we are going to grow the conservative movement, we’re going to take our message to the people who are struggling paycheck to paycheck,” he says on the stump. “To the students living under the burden of student loans, to the families struggling to raise their children with the right values, we will take our message to them and bring them to our side.”

Rubio demonstrated precisely how he would seek to achieve this at a hotel and casino on the southern end of the Las Vegas valley last week. On the morning of the Nevada caucuses, in the state where Rubio spent some of his childhood, he recounted that his parents had worked as a bartender and a maid in a structure much like the Silverton casino in which he stood.

“They park cars here at the valet and they sweep up the floors in the casino and they clean your room after you use it and they serve you your drinks and your food, and they work behind the tables,” Rubio said of hotel workers. “These people that are working here, they haven’t heard from conservatives in a long time.”

As Rubio concluded his speech, he earned a standing ovation and was mobbed by the crowd. Yet the exuberant scene made little difference in dictating the outcome of the fourth Republican contest that evening; another resounding win for Trump, with Rubio finishing a distant second.

It embodied in many ways Rubio’s greatest challenge: voters have shown little regard for electability, ideas or policy, but rather a preference for the sort of brutish invincibility personified by Trump.

Rob Damwijk, a tourist from San Diego who attended Rubio’s Las Vegas rally, reflected the contradiction near perfectly. The California native, who plans to vote in his state’s primary in June, said he was torn between Rubio and Trump.

“There were moments where I was moved to tears,” he said of Rubio. “He seems very committed and dedicated to average Americans.”

So why, then, was Damwijk considering Trump?

“He’s like the bad boy in class. And if you were going to be protected by somebody, that’s who you would stand behind.”

Rubio is making a mark, but he has yet to win a state. The senator believes he will win his home state of Florida. But even there, Trump holds a double-digit lead and a loss for Rubio on 15 March would probably be a fatal blow.

Reports have suggested Rubio’s campaign is preparing for a so-called brokered convention, which would in essence require Trump’s opponents to collude to rob him of the winning margin of 1,237 delegates required to secure the nomination. Though not out of the realm of plausibility, it would be a historic feat.

There are other remaining hurdles, too, should Rubio succeed. Democrats have eagerly tied the entire Republican field to Trump and his outlandish statements, particularly those directed towards immigrants.

During a recent conversation with reporters aboard his campaign bus, as Rubio reflected on his desire to expand the appeal of Republicans, The Guardian asked if he was concerned Trump’s tone had already alienated the groups whose support he would need to win an election.

“I think the party will largely be defined by who the nominee is,” Rubio said. “That’s why I’ve spent a lot of time talking about the plight of single women living on the edge of poverty, about empowering majority minority communities through education, and about student loan debt.”

Was he at least surprised by Trump’s success after having dismissed the reality TV star early on in his campaign as a sideshow?

“I don’t think anyone took him seriously at first,” Rubio said. “But I still don’t think he will be the nominee.”

Super Tuesday: day of decision

Super Tuesday is the biggest single night of the protracted process by which Republican and Democratic parties choose their nominees to run for president.

This week, 12 states plus American Samoa and Democrats Abroad will hold primaries or caucuses to dole out delegates among the remaining candidates.

So far, there has been a lot of excitement, but very few delegates pledged as a result of the early state contests. Super Tuesday changes all that.

The band of voting states, running from New England through the south and into the west, carry far more delegates than the early states of Iowa, New Hampshire, Nevada and South Carolina.

This year Super Tuesday is weighted towards southern states as Georgia, Virginia, Oklahoma, Alabama, Arkansas, Tennessee and Texas all vote.

For the Democrats, Hillary Clinton leads Bernie Sanders by 502 to 70, according to an Associated Press estimate that also tallies so-called super delegates, who are free to decide who to back at the July convention, regardless of the state-by-state primary results.

Clinton’s success with black voters in Nevada suggests she will do well in the south, while Sanders has captured young voters and will expect to push hardest in Massachusetts, Vermont, Colorado and Minnesota.

For Republicans, the quest to stop Trump may come down to a few key states. Texas native Cruz needs a big majority of his state’s 155 delegates, and Rubio will hope to cut through in cities and suburbs, for instance in Georgia and Virginia.

Super Tuesday’s delegates are given out proportional to results. On 15 March, the delegate-rich states of Ohio, Florida and Illinois will hold winner-take-all contests. But momentum from success on Super Tuesday could be decisive in both races.